A transmission system in which multi-level modulation using optical phase information is applied has been used in order to handle a recent rapid increase in communication link capacity. As a receiving system of the transmission system in which the multi-level modulation is applied, there is a digital coherent signal system in which a signal is received through a digital signal processing of a coherent signal detected by using interference between signal light and local oscillation light.
A Dual Polarization Quadrature Phase Shift Keying (DP-QPSK) system in which multiplexing is performed by assigning two phase signals to each of two lights having different polarizations each other is currently used for multi-level modulation. Using the transmission system in which the DP-QPSK system is applied, it is achieved to transmit a signal with a transmission capacity of 100 Gbps per one wavelength. A transceiver using such a transmission system has been specified as disclosed in Non Patent Literature 1 (“Multisource Agreement for 100G Long-Haul DWDM Transmission Module—Electromechanical”, Optical Internetworking Forum, Jun. 8, 2010) and Non Patent Literature 2 (“Implementation Agreement for Integrated Polarization Multiplexed Quadrature Modulated Transmitters”, Optical Internetworking Forum, Mar. 12, 2010).
In a typical transceiver specified in Non Patent Literatures 1 and 2, a CW light output from a transmission side light source is modulated with a transmission side electric signal using a multi-level modulator, and the modulated optical signal is transmitted to the outside as transmission light. At the same time, in such a transceiver, a light received from the outside and a local oscillation light output from a local oscillation light source are interfered using an interferometer and the interfered light is converted into a receiving side electric signal, with a photo diode. Herein, with the transceiver, in order to achieve further down-sizing and to reduce power consumption, a configuration in which the transmission side light source and the local oscillation light source are unified as one light source is considered. Such a configuration is an effective way of reducing power consumption and an installation size by unification of two wavelength controlling circuits, which were separated for transmission and receiving sides, respectively.
For example, a semiconductor Mach-Zehnder modulator is used as the multi-level modulator. The Mach-Zehnder modulator has a configuration in which one optical waveguide branches off to two optical waveguides, at an input side thereof, and the two branching optical waveguides are joined into one optical waveguide, at an output side thereof. An electrode is formed on each of the two branching optical waveguides. A bias voltage is applied to the electrodes by a bias control circuit, and a pair of complimentary voltage signals, which phase are opposite to each other, is also applied to the electrodes as a modulation signal by a driving circuit.
The bias voltage is set such that a phase difference between two lights passing through the two optical waveguide of the Mach-Zehnder modulator respectively is equal to π. In more detail, when the modulation signal is not input, the bias voltage is set such that a phase of light passing through one optical waveguide becomes 3π/2 and a phase of light passing through the other optical waveguide becomes π/2.
In a state in which the bias voltage is set in this way, when the modulation signal is input as a pair of complementary voltage signals, the phase of the light passing through the one optical waveguide is changed from 2π to π as is 3π/2+x (herein, −π/2≦x≦π/2), and the phase of the light passing through the other optical waveguide is changed from 0 to π as is π/2−x.
Further, a photo diode is put in at the output side of the Mach-Zehnder modulator. The photo diode receives a part of the transmission light branching from the Mach-Zehnder modulator as a monitoring light and detects an optical power of the transmission light. A bias control circuit controls the bias voltage to optimally maintain an operation point of the Mach-Zehnder modulator depending on the optical power detected by the photo diode.
Further, Japanese Patent Laid-Open Publication No. 2010-204689 has been known as a literature in which a technology relating to the corresponding technical field is disclosed.